This invention relates to improvements in track assemblies for tracked vehicles utilizing an endless track of chain-linked rigid pads trained about a pair of track sprockets. More particularly the invention relates to an improved structure for tensioning the idler track sprocket, and improved means for engaging the upper surface of the lower run of the track with a plurality of vehicle-supporting track rollers.
Tracked vehicles such as crawler tractors, designed to operate in slippery, muddy or otherwise difficult terrain where frictional engagement with the ground is normally insufficient to provide motive power, conventionally ride on a plurality of frame-mounted track rollers which engage the upper surface of the lower run of an endless track of chain-linked rigid pads trained about a pair of longitudinally spaced track sprockets. The rigid pads are normally of metal construction and have a transverse, outwardly facing cleat or "growser", usually of outwardly tapering cross-section, designed to penetrate the bearing surface of the terrain so as to resist slippage of the track with respect to the ground. One of the track sprockets (normally the rearward one) is rotatably driven through an axle and associated drive train by the vehicle engine, while the other or "idler" sprocket is not driven but is mounted so as to reciprocate in a direction longitudinal of the track assembly and is biased in a direction away from the driven sprocket so as to place tension continuously on the track and thereby prevent undue loosening thereof as the vehicle traverses uneven terrain.
Normally the device which provides the biasing of the idler sprocket is either a coil spring or hydraulic device interposed between a longitudinally movable journal box, which supports the hub of the idler sprocket, and the frame of the vehicle. Such biasing devices, while necessary to prevent undue loosening and resultant inadvertent detachment of the track during operation, have had certain disadvantages, notably with respect to cost and reliability. With regard to cost, the steel coil springs or hydraulic devices are required to be of such size as to add significant expense to the track assembly. With regard to reliability, such biasing devices sometimes permit the track to become too loose and disengage from the sprockets when the vehicle is being driven under a heavy load in a direction opposite to that where the idler sprocket is located, for example when a vehicle having a front-mounted idler sprocket is driven in reverse direction under high drive torque. This problem stems from the fact that, under such conditions, the driven sprocket transmits the driving force through the upper run of the track around the idler sprocket to the lower run whereas, when travelling in the opposite direction, the driving force is transmitted directly from the driven sprocket to the lower run of the track. When the drive force is transmitted to the upper run of the track and around the idler sprocket, it tends to pull the idler sprocket toward the driven sprocket. The conventional coil springs or hydraulic biasing devices, which tend to resist this movement, are sometimes unable to resist the high drive force without permitting excess longitudinal movement of the idler sprocket to the point where the track may become disengaged from the sprocket, thereby disabling the machine.
Another problem common to conventional metal-tracked vehicles concerns the manner in which the frame-mounted track rollers of the vehicle engage the upper surface of the lower run of the track. Normally the track rollers are shallow-grooved rollers having side flanges on either side of the groove which engage, in a partial straddling fashion, the flexible chain-link structure mounted on the rear sides of the pads. Since the chain links are much narrower than the pads, usually occupying a transversely central location on the backs of the pads, the engagement between the track rollers and the chain links constitutes substantially no more than a fulcrum engagement whereby the pads may easily tilt sideways in either direction with respect to the rollers. This type of engagement presents several problems, including the relatively high possibility of disengagement between the track and the rollers (thereby permitting the track to be "thrown"), a concentration of vehicle weight on the chain links instead of the pads tending to cause more frequent breakage of the links, and a rocking and twisting of the pads relative to the rollers causing instability and tending further to maximize the possibility of broken links.